This invention relates to lift trucks, and more particularly to "slip sheet" lift trucks in which loads are carried on sheets rather than pallets. Lift trucks of this type include push-pull mechanism which clamp onto a load-bearing sheet and pull the sheet onto a load carrying platform or platen at the front of the truck. For loading purposes the platen is tilted downwardly to a ground engaging position. When the load is to be unloaded, the push-pull mechanism pushes the load off the platen. U.S. Pat. No. 3,640,414 shows a typical mechanism of this type.
A continuing design problem with slip sheet lift trucks has been providing for pulling the sheet onto the platen at ground level. Unlike forklift trucks which handle palletized loads, and in which it is necessary only to get the forks low enough to slide into the pallets, in the present invention the forward end of the platen must go directly to the floor.
The prior art shows a number of devices of this general type. One type, for example, uses a lifting mast which is forwardly tilttable, so that the platen that extends therefrom can be tilted downwardly to a ground engaging loading position. However, when tilting a mast with a load in an elevated position, the center of gravity of the load and the mast moves, causing a substantial change in truck stability. Forward mast tilt causes substantial reduction of forward stability. Rearward mast tilt causes substantial reduction of lateral stability on trucks with three point suspension or narrow rear wheel spread. Also, tilting of the mast requires structural clearance which adds to the length of the truck. Further, tilting the mast with the platen or fork height above or below the mast tilt axis causes the platen or forks to move horizontally, and the truck must then be repositioned forward or reverse to maintain the same platen or fork position. Furthermore, typical prior art machines of this type have load supporting wheels beneath or rearward of the mast, so that the full cargo load is cantilevered forward of these wheels. This limits the load capacities of these trucks, and requires substantial counterweights on their rear ends.
Several slip sheet lift trucks are provided with outriggers forward of the mast to reduce the loading problem mentioned above. However, these outriggers interfere with and obstruct the platen when it is to be lowered to the ground. Therefore, in order to tilt the forward end of the platen downwardly to a ground engaging loading and unloading position, one such truck jackknifes in the middle. Another has small outrigger wheels on arms which are pivoted for vertical movement between carrying and loading positions. Mechanisms such as these are complicated and expensive.
A need thus remains for an uncomplicated and inexpensive slip sheet lift truck which has the advantages of outrigger forward support without the expensive and complicated mechanisms of the prior art, and which can be equipped with a lifting mast which avoids the problems discussed above.